71M6545 datasheet(13/133 Pages) MAXIM

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71M6545/71M6545H Data Sheet

The 71M6545/H AFE can also be directly interfaced to Current Transformers (CTs), as seen in Figure 3.

In this case, all voltage and current channels are multiplexed into a single second-order sigma-delta ADC

in the 71M6545/H and the 71M6xx3 remote isolated sensors are not used. The fourth CT and the

measurement of Neutral current via the IADC0-IADC1 current channel are optional.

See Figure 28 for the meter wiring configuration corresponding to Figure 3.

∆Σ

ADC

CONVERTER

VREF

MUX

VREF

VADC

FIR

IADC4

VADC9 (VB)

IADC2

VADC8 (VA)

IADC3

IADC5

71M6545/H

CE RAM

IADC6

IADC7

IADC0

IADC1

IN*

VADC10 (VC)

10/7/2010

*IN = Neutral Current

Figure 3. AFE Block Diagram (Four CTs)

2.2.1 Signal Input Pins

The 71M6545/H features eleven ADC input pins.

IADC0 through IADC7 are intended for use as current sensor inputs. These eight current sensor inputs can

be configured as four single-ended inputs, or can be paired to form four differential inputs. For best

performance, it is recommended to configure the current sensor inputs as differential inputs (i.e., IADC0-

IADC1, IADC2-IADC3, IADC4-IADC5 and IADC6-IADC7). The first differential input (IADC0-IADC1)

features a pre-amplifier with a selectable gain of 1 or 8, and is intended for direct connection to a shunt

resistor sensor, and can also be used with a Current Transformer (CT). The three remaining differential

pairs (i.e., IADC2-IADC3, IADC4-IADC5 and IADC6-IADC7) may be used with CTs, or may be enabled to

interface to a remote 71M6xx3 isolated current sensor providing isolation for a shunt resistor sensor using a

low cost pulse transformer.

The remaining three inputs VADC8 (VA), VADC9 (VB) and VADC10 (VC) are single-ended, and are

intended for sensing each of the phase voltages in a poly-phase meter application. These three single-

ended inputs are referenced to the V3P3A pin.

All ADC input pins measure voltage. In the case of shunt current sensors, currents are sensed as a voltage

drop in the shunt resistor sensor. In the case of Current Transformers (CT), the current is measured as a

voltage across a burden resistor that is connected to the secondary of the CT. Meanwhile, line voltages are

sensed through resistive voltage dividers. The VADC8 (VA), VADC9 (VB) and VADC10 (VC) pins are

single-ended and their common return is the V3P3A pin. See Figure 23, Figure 24, Figure 25 and Figure

26 for detailed connections for each type of sensor.

Pins IADC0-IADC1 can be programmed individually to be differential or single-ended as determined by

the DIFF0_E (I/O RAM 0x210C[4]) control bit. However, for most applications, IADC0-IADC1 are

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